The basic cube-corner retroreflective element used in cube-corner retroreflective articles has a notoriously low angularity, i.e., the element will only brightly retroreflect light that impinges on it within a narrow angular range centering approximately on its optical axis. Low angularity arises by the inherent nature of these elements, which are trihedral structures having three mutually perpendicular lateral faces, such as occur at the corner of a cube. In use, the elements are arranged so that light to be retroreflected impinges into the internal space defined by the faces, and retroreflection of the impinging light occurs by internal reflection of the light from face to face of the element. Impinging light that is inclined substantially away from the optical axis of the element (which is the trisector of the internal space defined by the faces of the element) strikes a face at an angle less than its critical angle, thereby passing through the face rather than being reflected.
Some workers have addressed this problem by coating the trihedral faces of the cube-corner element with specularly reflective metal, e.g., vapor-deposited aluminum, to cause even highly inclined light to be reflected by the faces. But such coatings reduce overall reflection from the faces (because a percentage of light impinging on the faces is absorbed by the coating), and introduce a gray color to the element which is often objectionable. Also the coatings could be susceptible to corrosion problems that would limit the useful life of an article having such elements.
Others have addressed the problem by arranging a second retroreflective plate or sheet in back of a first retroreflective plate or sheet (see Weber, U.S. Pat. No. 3,140,340, McGrath, U.S. Pat. No.4,025,159, or Jones, U.S. Pat. No. 4,303, 305), but such an approach is expensive and provides a thick and generally rigid construction not suited for many uses.
Others have addressed the problem by attempting to change the configuration of the cube-corner retroreflective elements, but none of these efforts has provided a practically manufactured sheeting suitable for the most common uses of retroreflective sheeting (e.g., on traffic signs, license plates, advertising signs, etc.). For example, White, U.S. Pat. No. 4,349,598 teaches a limited-use broader-angularity retroreflective sheeting obtained by tilting the central axes of the cube-corner elements to an approximately 35-degree angle and joining two adjacent elements into a right triangle prism or "pup-tent"-like configuration. Such cube-corner retroreflective elements achieve retroreflection of light having incidence angles (the angle between the incident light and a line perpendicular to the sheeting) approaching 90.degree., which makes them useful particularly for pavement markings or the like. But 0.degree.-incidence angle light (light that is perpendicular to the sheeting) is not reflected, and accordingly the sheeting is not useful for conventional traffic signs.
The desire for a thin pliable cube-corner retroreflective sheeting that would reflect inclined light is recognized in Haggerty, U.S. Pat. No. 3,450,459, but the patent teaches no practical method for achieving such a result. Thin pliable sheeting requires that the cube-corner elements be of very small "microsizes," which so far as known, have only been accomplished by grooving techniques for which the elements taught in Haggerty are not adapted.
Prior efforts have also been made to increase the angular range of reflector plates that use larger cube-corner retroreflective elements, such as the reflector plates mounted on vehicles. The molds for such reflector plates are generally made by bundling together individual mold parts, typically pins which each have an end portion shaped like a cube-corner, retroreflective element. Heenan et al, U.S. Pat. No. 3,541,606, teaches reflector plates for vehicles containing cube-corner retroreflective elements arranged into discrete, rather large groups. The optical axes of the cube-corner elements in each group are inclined at angles different from the angles of the elements of a different group so as to increase the angular range of reflection in a horizontal plane around a vehicle. Heenan, U.S. Pat No. 3,923,378 and Heenan, U.S. Pat. No. Re: 29,396 teach an improvement in which the cube-corner retroreflective elements are arranged in rows, and the optical axes of the cube-corner elements of one row are inclined towards the elements of the other row, for example, in an amount between about 6.degree. and 13.degree. (see column 5, line 45 et seq. of U.S. Pat. No. 3,923,378). Such tilting is intended to increase the angularity of the reflector plate in a predetermined plane (see column 5, lines 64 and 65), which is typically the horizontal plane around a vehicle in other embodiments (see FIGS. 19 and 31 of either U.S. Pat. No. Re. 29,396 or U.S. Pat. No. 3,923,378), increased angularity is obtained in two planes by mixing two different sets of cube-corner retroreflective elements, one set comprising cube-corner elements inclined towards one another in one of the planes of desired increased angularity, and the other set comprising cube-corner elements that are inclined towards one another in the other of the planes of desired increased angularity. Lindner, U.S. Pat. No. 4,066,331, has a similar objective with cube-corner retroreflective elements arranged in rows.
The noted improvement as to the angularity of reflector plates in one plane, such as the horizontal plane around a vehicle, has less value for other kinds of retroreflective articles. Retroreflective sheeting, in particular, is generally intended for use on large-area surfaces that are viewed over their whole surface and from many angles. For reflective sheeting used on sign faces it is important to maintain a uniform brightness over the whole surface of the sheeting irrespective of the viewing angle, so that the whole sign has uniform brightness, and so that the legends or symbols on the sign are legible. Legibility requires control of the contrast between the graphic images and background area of the sign and such control of contrast requires uniformity in reflective brightness over the whole viewing surface.